Method and device for monitoring a flame

ABSTRACT

The method according to the invention of monitoring a flame uses the known principle that a direct current signal (I F ) of different magnitude is produced in dependence on the presence or intensity of the flame from an ac voltage signal. That purpose is served for example by ionization electrodes ( 3 ) or ultraviolet sensors ( 4, 4   a ) which in dependence on the intensity of the flame produce a corresponding direct current signal. When the flame is extinguished no direct current signal is produced. The direct current signal (I F ) is detected by an evaluation circuit ( 6 ) and converted into a first output signal (A), wherein conversion is effected by various further circuit elements ( 7, 9, 10 ) in such a way that differently changing output signals (A 1 , A 2 ) are obtained depending on the respective flame intensity. The evaluation circuit ( 6 ) also is acted upon by an ac voltage signal whose absence, upon failure of the evaluation circuit ( 6 ), deactivates a monitoring circuit ( 7 ), which results in a second output signal (A A ). That second output signal (A A ) is advantageously a static output signal so that it is possible to detect defective performance on the part of the evaluation circuit ( 6 ). The function of the monitoring circuit ( 7 ) itself can be periodically tested.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method of monitoring a flame such as a flame ofa gas or oil burner and an apparatus for monitoring a flame.

2. Description of the Prior Art

Monitoring gas flames frequently entails the use of flame monitors whichutilize the rectifier effect of the flame, that is to say which operateon the basis of what is known as the ionization principle. In thatprocedure an ac voltage is applied between two electrodes. The volumewhich is filled by the flame depends on the instantaneous output of theburner. The direct current which can be produced can be very low at alow level of burner output and if the geometry of the electrodes is notthe optimum, while the alternating current can be substantially greaterin dependence on the capacitance of the sensor line. The flame signalamplifier must therefore be capable of filtering off the low directcurrent component in the overall sensor circuit current, without thealternating current being able to simulate a flame signal as a result ofthe inevitable rectifier effects in the amplifier input. Therefore themagnitude of the direct current component gives a measurement in respectof the intensity of the flame, in which respect the absence of a flamecorresponds to the intensity of zero, the detection of which must beestablished reliably and very close to real time in order to avoidunburnt gas or oil from flowing out into the burner chamber.

In principle filtering of the direct current component can beimplemented by an evaluation circuit which is connected upstream of theflame signal amplifier, such as for example a low pass filter with asufficiently low limit frequency. If however the filter capability ofthe low pass filter is lost, for example because of a failure of afilter capacitor, the alternating current could simulate the presence ofa flame, even when the flame is not present. The flame monitoring orburner control system must recognise that malfunction. In the case ofburners involving intermittent operation, that is normally not a problembecause, after the fuel supply is switched off, which results in theflame being extinguished, the control system can detect a simulatedflame signal as being a defect and can prevent the burner from being setin operation again. In the case of burners in continuous operation themalfunction must be detected by periodically checking the flame monitorwithout the burner being taken out of operation for that purpose. In thecase of optical flame sensors, this is generally effected byinterrupting the beam path between the flame and the sensor by means ofa shutter member, that is to say flame failure is temporarily simulatedduring operation of the assembly, and the output of the flame signalamplifier must suitably react thereto.

In principle the method of signal interruption at the flame sensor canalso be used in regard to ionization flame monitoring. The ionizationcircuit could be interrupted by means of a suitable switch element.However that element would have to be disposed close to the sensorelectrode so that only the flame signal current is interrupted and notfor example also the alternating current which flows by way of linecapacitances and whose flame-simulating effect in the event of acomponent fault is in fact precisely to be detected by the test. Itwould also be possible to envisage short-circuiting of the flame signallines whereby the flame signal current also becomes zero, and thealternating current is even increased. For both cases, it would benecessary to use a switch element which is suitable for the high sensorac voltage and which itself cannot assume a fault mechanism whichresults in undetected flame simulation.

In the present state of knowledge only an electromechanical relay can beconsidered for that purpose. That structure however is expensive interms of material and equipment and requires a relatively high level ofcontrol power. The possibility of interrupting the sensor current with arelay contact is mentioned in German laid-open application (DE-OS) No 2932 129 on page 6. DE 30 26 787 describes a construction in which thereis a single filter capacitor at the input of the flame signal amplifier,which on the one hand serves as an energy storage means for theionization current and whose discharge current on the other hand isrequired for dynamic operation of a semiconductor circuit. Failure ofthat filter capacitor has the consequence that the semiconductorcircuit—even in the case of an alternating current caused by sensor linecapacitances—goes into a constant state so that a flame is no longersignalled. The disadvantage of that arrangement is that a given minimumlevel of energy and thus a given minimum current must be supplied by theflame for dynamic operation of that semiconductor circuit. Thereforecertain limits are set on the response sensitivity of that circuitryprinciple and it no longer satisfies all present-day requirements.

EP 159 748 discloses a circuit which leads to the assumption of a highlevel of response sensitivity insofar as the capacitive load currentcaused by line capacitances, at the sensor terminals, remains low incomparison with the flame signal current. In that respect this circuitdoes not satisfy the demands for a high level of response sensitivityand at the same time a high level of resistance in relation to linecapacitance. A further requirement which is frequently specified is thedisplay of flame intensity as a setting aid when bringing a burner intooperation and for detecting changes of the flame in operation, in goodtime. The circuit disclosed in EP 159 748 does not afford that option.

The arrangement in accordance with the teaching of DE 30 26 787 suppliesa pulse series in dependence on the magnitude of the flame current sothat in that case it would be possible to derive a signal to indicatethe flame intensity, but the dynamic range between response sensitivityand saturation limit is relatively small so that the circuit principleis only suitable for establishing “flame present”.

EP 0 617 234 also discloses an ionization flame monitor with a circuitarrangement having a capacitor which is transferred by the ionizationcurrent from a condition of being charged up by the operating voltageinto a discharged condition, wherein the signal “flame present” isoutput when the value falls below a given threshold. The function of thecapacitor can be checked by means of a test signal. The disadvantagehere is that the function of the capacitor has to be periodicallytested, the system does not provide for continuous monitoring of thecapacitor.

SUMMARY OF THE INVENTION

Therefore the object of the present invention is to provide a method andan apparatus for monitoring a flame which serves as a flame monitoringmethod and circuit respectively, the response sensitivity of which issubstantially improved in comparison with the state of the art withoutdetracting from compatibility for line capacitance, whose switch-offcapability can be periodically checked during burner operation and alsosupplies an output signal representing a measurement in respect of flameintensity. The invention further seeks to provide that the methodensures continuous checking of the monitoring action.

According to a first aspect of the present invention, there is provideda method of monitoring a flame, wherein:

in dependence on the presence or intensity of the flame there isproduced from a first electrical signal a second electrical signal ofdifferent magnitude,

the second electrical signal is applied to an evaluation circuit andconverted into a first output signal, and

the evaluation circuit is acted upon by a monitoring signal which uponfailure of the evaluation circuit leads to a second output signal.

According to a second aspect of the present invention, there is providedapparatus for monitoring a flame, comprising:

an evaluation circuit which in dependence on the presence or theintensity of the flame generates from a first electrical signal a secondelectrical signal of different magnitude, and

circuit means which converts the second electrical signal into a firstoutput signal,

wherein the evaluation circuit can be acted upon with a monitoringsignal which in the event of failure of the evaluation circuit leads toa second output signal.

Advantageous configurations are set forth in the dependent claims.

The method according to the invention of monitoring a flame makes use ofthe known principle that in dependence on the presence or the intensityof the flame there is produced from a first electrical signal (forexample an ac voltage signal) a second electrical signal of differentmagnitude (for example a dc signal) (I_(F)) . A preferred embodimentuses ionization electrodes or ultraviolet sensors with series-connecteddiode which in dependence on flame intensity supply a corresponding dcsignal. No dc signal is produced when the flame is extinguished. Thesecond electrical signal (I_(F)) is detected by an evaluation circuit towhich the ionization electrodes or the ultraviolet sensors areconnected, and converted into a first output signal (A), whereinconversion is effected by various further circuit elements in such a waythat differently dynamic output signals are obtained, depending on therespective flame intensity involved. Therefore with changing flameintensities the output signal (A) is an output signal which changes interms of its dynamics.

The evaluation circuit is also acted upon by an electrical monitoringcircuit (ac voltage signal) which can be derived for example from the acvoltage signal made available to the ionization electrodes, which uponfailure of the evaluation circuit results in a second output signal(A_(A)). That second output signal, as in the case of flame failure, isadvantageously a static signal, so that the monitoring apparatusimmediately notes the failure of the evaluation circuit and can causethe fuel supply to be shut off.

The second electrical signal (I_(F)) is converted into a control signal(S) and passed on to a trigger stage. This trigger stage can be forexample an operational amplifier which compares the control signal to agiven threshold and then resets the evaluation circuit again by way of areset signal (R) so that it can again control the trigger stage. In thatway the output of the trigger stage is switched over between two outputsignals (A₁, A₂) in dependence on the control signal (S). The triggerstage switches to and fro at different rates in dependence on the flameintensity.

The control signal (S) can also be passed by way of a further evaluationcircuit in order to improve the sensitivity of the circuit in relationto the second electrical signal, that is to say for example in relationto the direct current component in the sensor current. In order to checkthat second evaluation circuit, that is to say in order to detect afailure, its circuitry is connected to the control input of anintegrator which is in the form of a charge pump and whose output signalreflects the magnitude of the second electrical signal, such as forexample of the sensor current.

A monitoring circuit serves to detect a failure of the first evaluationcircuit, the monitoring circuit being supplied by way of the evaluationcircuit with a monitoring signal, that is to say for example an acvoltage signal, so that in the event of failure of the evaluationcircuit the monitoring circuit is taken out of operation and thatresults in a static output signal (A_(A)). The output signal of theintegrator becomes zero in the event of failure of the sensor current.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described in greater detailhereinafter with reference to the drawings in which:

FIG. 1 is a diagrammatic view of the flame monitoring circuit,

FIG. 2 shows a block circuit diagram of the flame monitoring circuit,

FIG. 3 shows a detailed circuit diagram of the flame monitoring circuit,

FIG. 4 shows a development of the flame monitoring circuit, and

FIG. 5 shows three time diagrams of the direct current signal, thefailure test and the output signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagrammatic view of a preferred embodiment of apparatusaccording to the invention. Ionization electrodes 3 or ultravioletsensors 4,4 a are supplied by way of a connecting terminal 1 with the acvoltage signal from a suitable source 5 and supply the signal which isgenerated by the flame and on which an unwanted alternating currentsignal is superimposed to the terminal 2 at which an evaluation circuit6, here a filter member, detects the direct current signal I_(F). Thecontrol signal S is passed to the trigger stage 9 which outputs theoutput signal A, A_(A). A reset line R serves to reset the evaluationcircuit 6 so that an oscillating signal appears at the output of thetrigger stage 9. If the evaluation circuit 6 comprises a low pass memberTP with capacitor C1 and resistor R1, it has to be regularly reset.

The ac voltage source 5 also feeds the evaluation circuit 6 whichtransmits the monitoring signal, that is to say the ac voltage of the acvoltage source 5, to a monitoring circuit 7, here a charge pump, whichputs the trigger stage 9 into a given condition which activates thetrigger stage 9. Upon failure of the evaluation circuit 6 no signal istransmitted to the monitoring circuit 7 so that the trigger stage 9 isput into another static condition which interrupts further signalling ofthe flame intensity (output signal A) and then has the output signalA_(A). In that way failure of the evaluation circuit 6 can be easilydetected. A test signal T can be applied to a switch 11 which simulatesfailure of the evaluation circuit 6. It is thus possible once again tocheck the circuit for failure detection in respect of the evaluationcircuit 6, in particular the charge pump and the trigger stage 9.

FIGS. 2 and 3 show a block circuit diagram and a detailed circuitdiagram respectively of the flame monitoring circuit of the preferredembodiment. The circuit diagram shows the components with the usualsymbols and the usual references. The precise wiring configurationinvolved will not be described in detail here, as it can be seen fromFIGS. 2 and 3. The flame monitoring circuit is fed in bipolar mode bytwo operating voltages +Ub1 and −Ub2 defined with respect to a referencepotential m. It has two terminals 1 and 2 which can be connected eitherto two ionization electrodes 3 or to the two terminals of an ultravioletsensor comprising a gas-filled ultraviolet cell 4 and a diode 4 aconnected in series therewith. The first terminal 1 serves as an outputwhich carries an ac voltage which is produced by an ac voltage generator5 and which is defined with respect to the reference potential m. Thesecond terminal 2 serves as an input to which the actual sensor signalis fed. Connected downstream of the second terminal 2 is a first lowpass member 6 formed from a resistor R1 and a capacitor C1. The acvoltage produced by the ac voltage generator 5 is taken by way of alimiting resistor R3 and a coupling capacitor C3 to the capacitor C1 andto the input of a charge pump. The signal at the output of the chargepump is taken by way of a voltage divider 8 connected to the positiveoperating voltage, to the non-inverting input of an operationalamplifier 9 which is connected as a Schmitt trigger. The inverting inputof the operational amplifier 9 is connected to the output of the lowpass member 6. The output of the operational amplifier 9 controls aswitch 10 by way of which the capacitor C1 can be discharged.

The ac voltage which acts on the capacitor C1 and which in theillustrated example is derived from the ac voltage generated by the acvoltage generator 5 could also be generated by a second ac voltagegenerator.

Only a direct current flows in the sensor circuit between the ionizationelectrodes 3 because of the rectifying effect of the flame or in theultraviolet cell 4 because of the diode 4 a, more specifically only whenthe flame is actually burning. However an unwanted alternating currentalso constantly flows between the terminals 1 and 2, because of theinevitable capacitance of the sensor lines, and that alternating currentis superimposed on the direct current. The flame monitoring circuit isnow designed in such a way that this alternating current is notrectified and therefore cannot simulate a signal “flame present” whenthe flame is missing.

The flame monitoring circuit operates as follows: as long as thecapacitor C1 is intact, the charge pump 7 carries at its output anapproximately constant negative potential U_(C5), whose absolute valueis about 75-80% of the positive feed voltage +Ub1. The sizes of theresistors R7 and R8 of the voltage divider 8 are such that the voltageat the non-inverting input of the operational amplifier 9 is alsonegative. The output of the operational amplifier 9 firstly carries thenegative operating voltage −Ub2 so that the switch 10 which is in theform of a junction field effect transistor T2 is open. As soon as theflame is present the direct current flowing between the ionizationelectrodes 3 or the photoelectric current of the ultraviolet sensor 4charges up the capacitor C1 whose potential becomes increasingly morenegative. As a consequence the voltage at the inverting input of theoperational amplifier 9 also falls to an increasingly negativepotential. As soon as the voltage at the inverting input of theoperational amplifier 9 falls below the voltage at the non-invertinginput of the operational amplifier 9, the output of the operationalamplifier 9 carries the positive feed voltage +Ub1, the switch 10 closesand the capacitor C1 begins to discharge. Because of the resistors R5and R6 the operational amplifier 9 has a certain switching hysteresis sothat the capacitor C1 is partially discharged. When discharging of thecapacitor C1 has progressed to a sufficient degree, the output of theoperational amplifier 9 then switches over again and again carries thenegative feed voltage −Ub2. The cycle thus begins again. The signal atthe output of the operational amplifier 9 is a rectangular signal. Thefrequency thereof represents a measurement in respect of flame intensityas the strength of the direct current flowing between the ionizationelectrodes 3 determines the period of time which it takes to charge upthe capacitor C1 until the operational amplifier 9 switches over again.

An interruption in the capacitor C1 has the result that the transistorT1 of the charge pump 7 is continuously non-conducting and the chargepump 7 is therefore out of operation. Consequently the capacitor C5 ischarged up to the positive feed voltage Ub1 so that the output of thecharge pump 7 and also the output of the operational amplifier 9 carry astatic signal. A short-circuit of the capacitor C1 has the result thatthe charge pump 7 admittedly remains in operation, but the amplitude ofthe voltage at the inverting input of the operational amplifier 9remains sufficiently small, in relation to the voltage at thenon-inverting input, so that the output of the operational amplifier 9again carries a static signal.

Only an alternating signal at the output of the operational amplifier 9therefore means that the flame is present, while a uniform signal meanseither that the flame is not burning or that the flame monitoringcircuit is defective.

With the proposed flame monitoring circuit, the amplitude of the acvoltage produced by the ac voltage generator 5, the resistor R3 and thecapacitors C1 and C3 must be matched to each other in such a way thatthe amplitude of the ac voltage at the capacitor C3 and thus also at theinverting input of the operational amplifier 9 is not sufficient tocause the operational amplifier 9 which is connected as a Schmitttrigger to switch backward and forward and thus simulate a “flamepresent” signal.

In intermittent operation of the burner the flame monitoring circuit canbe checked, whenever the burner is switched off, to ascertain whether no“flame present” signal appears at the output. In the case of a flamemonitoring circuit which is suitable for continuous burner operation,there is a second switch 11 with which the input of the charge pump 7can be connected to the reference potential m. When the switch 11 isclosed then the information “flame not present” must appear at theoutput of the flame monitoring circuit and/or downstream-disposedcircuits. The switch 11 is preferably operated by a microprocessor. Theswitch 11 shown in FIG. 3 is an optocoupler which is controlled by wayof two inputs and which permits galvanically separated control.

FIG. 4 shows a development of the flame monitoring circuit in whichconnected between the capacitor C1 and the input of the operationalamplifier 9 is a second low pass member 19 formed from a resistor R2 anda capacitor C2. In this case the switch 10 controls discharging of thecapacitor C2. In a similar manner to the capacitor C1 the capacitor C2must be monitored for a possible interruption. The capacitor C2 istherefore connected to the input of an integrator 20, at the output ofwhich there is a dc voltage whose level is a measurement in respect offlame intensity. The integrator 20 is in the form of a charge pump. Thecapacitor C7 is recharged in accordance with the frequency of thecharge/discharge cycles of the capacitor C2 by way of the capacitor C6.The frequency is determined by the sensor current. In the event of aninterruption of the capacitor C2 the voltage at the capacitor C7 assumesthe value of the reference potential m, which is equivalent to “flamenot present”. The voltage at the capacitor C7 is digitized for exampleby means of a voltage/frequency converter and transmitted ingalvanically separated fashion by way of an optocoupler to a superioritem of equipment, for example an automatic firing assembly. Theadvantage of this circuit is that the low pass member 19 attenuates theac voltage produced by the ac voltage generator 5, in such a way that asubstantially greater ratio can be accepted between the alternatingcurrent caused by the sensor line capacitances and the ionizationcurrent.

If the flame is monitored with a UV-cell 4 which in contrast to theionization electrodes 3 is not fail-safe because there is the dangerthat the UV-cell 4 fires for example as a result of ageing even when aflame is not present, the system comprising the UV-cell 4 and the flamemonitoring circuit must be tested in continuous operation of the burnerby blacking out the UV-cell 4. The switch 11 may not be operated then.It can be omitted if the flame monitoring circuit is to be used onlywith UV-cells 4.

FIG. 5 shows time diagrams in respect of the signals shown in FIGS. 1and 2. The uppermost diagram shows the direct current signal I_(F) onwhich the alternating current signal is superimposed, in which case thealternating current signal is only shown in part for the sake ofenhanced clarity. The flame begins to burn at the time t₁, and it ispossible to see a direct current signal which rises to the time t₂.Until t₃ the flame intensity remains constant and then falls to t₄ inorder there to remain at a lower level in order finally to rise againfrom the time t₅ and remain at a higher level from time t₆.

The lowermost diagram plots the output signal A which switches to andfro between the two limit values of the trigger stage 9 or theoperational amplifier A₁=+Ub1, A₂=−Ub2. Up to the time t₁ there is nocharging of C1 and the amplifier output remains at A₂. After the flameproduces a direct current the capacitor C1 of the low pass filtercharges up and after a certain charging time causes the trigger stage 9to switch over. Between the times t₂ and t₃ the change-over switchingtime t_(u) is approximately constant so that a given frequency f₁ isset, representing a measurement in respect of flame intensity. Betweenthe times t₄ and t₅ there is a frequency f₂ and from t₆ there is thefrequency f₃. Each of the frequencies is therefore associated with oneof the direct current signals I_(F1), I_(F2) and I_(F3).

Shown in the middle of the diagrams is the test signal T which isapplied between the times t₇ and t₈. When the charge pump 7 isoperating, that results in fixing the potential of an input of theamplifier so that—when the trigger stage is functioning—there is nolonger any change-over switching action. That can be seen in the outputsignal diagram between the corresponding times, with a minor time lag.That output signal A_(A) therefore shows that the circuit is intact,that is to say the circuit can be tested even in uninterrupted operationof the burner. Without the test signal T the signal A_(A) signalsabsence of the flame.

I claim:
 1. A method of monitoring a flame, wherein: in dependence onthe presence of intensity of the flame there is produced by means of asensor from a first electrical signal supplied to the sensor a secondelectrical signal of different magnitude, and the second electricalsignal is applied to an evaluation circuit and converted into a firstoutput signal, wherein a monitoring circuit is acted upon by amonitoring signal by means of the evaluation circuit so that in theevent of failure of the evaluation circuit a second output signal isproduced by means of the monitoring circuit, and wherein the monitoringcircuit is a charge pump and the monitoring signal is an ac voltagesignal so that in the event of failure of the evaluation circuit thecharge pump is taken out of operation and that results in the secondoutput signal being produced as the second output signal.
 2. A method asset forth in claim 1, wherein the first output signal is an alternatingsignal and the second output signal is a uniform or static signal.
 3. Amethod as set forth in claim 1, wherein the second electrical signal isa direct current signal and is converted by the evaluation circuit intoa control signal and passed to a trigger stage, the trigger stage isswitched over in dependence on the control signal between two outputsignals, and different change-over switching times occur between theoutput signals in dependence on the magnitude of the direct currentsignal.
 4. A method as set forth in claim 3, wherein the control signalis applied to a further evaluation circuit and integrated by means of anintegrator, and the integrated signal is either converted by means of avoltage-frequency converter into the first output signal or itselfserves as the first output signal.
 5. A method as set forth in claim 4,wherein the integrated signal of the integrator results in an outputsignal of zero in the event of failure of the further evaluationcircuit.
 6. A method as set forth in claim 1, wherein the first outputsignal is used to indicate the intensity of the flame.
 7. A method asset forth in claim 1, wherein the monitoring circuit is checked byapplying thereto a test signal to check that the second output signal isproduced.
 8. Apparatus for monitoring a flame, comprising: an evaluationcircuit which in dependence on the presence or the intensity of theflame generates be means of a sensor from a first electrical signalsupplied to the sensor a second electrical signal of differentmagnitude, circuit means which converts the second electrical signalinto a first output signal, and a monitoring circuit configured to beacted upon with a monitoring signal by means of the evaluation circuitso that in the event of failure of the evaluation circuit, a secondoutput signal is produced by means of the monitoring circuit, whereinthe evaluation circuit is a low pass member having a resistor and acapacitor, the monitoring signal is an ac voltage, the monitoringcircuit is a charge pump, and the capacitor can be acted upon by the acvoltage in order to produce the second output signal as the secondoutput signal in the event of failure of the capacitor.
 9. Apparatus asset forth in claim 8, wherein the ac voltage with which the evaluationcircuit can be acted upon can be applied to the input of the chargepump, and the charge pump has an output which carries an approximatelyconstant potential of a first polarity when an alternating signal occursat the input thereof and which carries a constant potential of a secondpolarity when a uniform or static signal occurs at the input thereof, sothat the signal at the output of the charge pump indicates whether theevaluation circuit is defective.
 10. Apparatus as set forth in claim 8,wherein the capacitor of the evaluation circuit is connected to thefirst input of an operational amplifier connected as a Schmitt trigger,the potential at the output of the charge pump controls the potential atthe second input of the operational amplifier, and the output of theoperational amplifier controls a switch by way of which the capacitorcan be discharged so that a rectangular signal appears at the output ofthe operational amplifier when a flame is present.
 11. Apparatus as setforth in claim 10, wherein connected between the capacitor and the firstinput of the operational amplifier is a further evaluation circuithaving a second resistor and a second capacitor, and the secondcapacitor is connected to the input of an integrator, and at its outputthe integrator carries an approximately uniform voltage whose level is ameasurement of the frequency with which the second capacitor isdischarged through the switch.
 12. Apparatus as set forth in claim 8,wherein there is a switch with which a test signal “flame not present”can be applied to the input of the monitoring circuit so that even whena flame is present it is possible at any time to check whether theindividual circuit means of the apparatus are operating correctly.